The present invention relates to a rotisserie type fixture to be used for coating workpieces, such as airfoils and vanes for industrial turbines and jet engines.
Airfoils, such as vanes, used in industrial turbines and jet engines are typically subjected to a coating operation to improve their resistance to corrosion and thermal fatigue, and to otherwise enhance their performance. Economic concerns dictate that one must be able to simultaneously coat multiple airfoils in order to have a viable commercial process.
Fixtures for use in PVD coating processes for airfoils and vanes are known. Typically, such fixtures provide only rotation of the airfoils, which are immovable and horizontally placed in the fixture around the axis of a horizontal shaft. The disadvantages of such fixtures are poor adhesion of coatings on surfaces and unequal thickness of coatings on such surfaces.
There are also well known fixtures which provide simultaneous rotation of workpieces being coated around their own axis and around the axis of the shaft. U.S. Pat. No. 4,108,107 to Scheuerman illustrates one such fixture. Unfortunately, such fixtures can not provide airfoils having platform and shroud surfaces with coatings having good adhesion properties. Additionally, the fixtures have a very complicated construction with a planetary mechanism and twin shaft constructions.
One commercial system for simultaneously coating a plurality of workpieces is shown in U.S. Pat. No. 4,192,253. In this system, a plurality of workpieces to be coated are inserted into a coating chamber and simultaneously rotated about their longitudinal axes during the coating operation. The simultaneous rotation of the workpieces around their longitudinal axes is accomplished by mounting each workpiece to the tip of a fixture whose other end is connected to a complex gear operated drive system for rotating the fixture and the individual workpieces mounted thereto. This type of system suffers from the problems previously discussed. For example, the system is complex and potentially troublesome from a maintenance standpoint. Still further, the costs associated with such a system are quite high. Yet another disadvantage is the inability of the system to provide good quality coatings on surfaces of an airfoil.
More recently, it has been proposed to coat multiple workpieces at a single time using a single modular fixture arrangement. Such a coating system is illustrated in copending, allowed U.S. patent application Ser. No. 08/782,398, filed on Jan. 13, 1997, to John W. Menchetti et al., entitled MODULAR COATING FIXTURE, assigned to the assignee of the instant application.
Certain coating techniques, such as electron beam physical vapor deposition (EB-PVD), are line of sight techniques wherein surfaces of a workpiece which are not in a line of sight with the source of the coating material will not be coated. Efforts have been made to create coating fixtures which place all surfaces which require coating in a line of sight with the coating source. These efforts are illustrated in copending U.S. patent application Ser. No. 08/813,384, filed on Mar. 6, 1997, to Steven M. Burns et al., entitled MODULAR COATING FIXTURE, and Ser. No. 08/895,980, filed on Jul. 17, 1997, to Steven M. Burns et al., entitled VARIABLE TILTING TUMBLER VACUUM COATING APPARATUS, both assigned to the assignee of the instant application.
Unfortunately, the prior art fixtures are not universal. In other words, they may not be used to coat all airfoils and vanes. Thus, there remains a need for customized coating fixtures which facilitate the fabrication of substantially uniform coatings along the surfaces of workpieces and which are economically viable.
The coating fixture of the present invention meets the foregoing requirements and needs.